Molecular profile and clinicopathologic features of follicular variant papillary thyroid carcinoma

  • Background:
    The non-invasive encapsulated follicular variant of papillary thyroid carcinoma (FVPTC) has an indolent clinical behavior. Recently, it was proposed that this tumor type should be reclassified as non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP). To characterize NIFTPs, we evaluated the molecular and clinicopathologic characteristics of each FVPTC subtype.

    Methods: This study enrolled 29 patients with FVPTC who underwent thyroidectomy between January 2007 and June 2017. They were classified as non-invasive encapsulated FVPTC (NIFTP, n = 10), invasive encapsulated FVPTC (n = 11), and infiltrative FVPTC (n = 8) by two independent pathologists. Genetic alterations were analyzed by targeted next-generation sequencing using formalin-fixed, paraffin-embedded tissue samples and the clinicopathologic characteristics were retrospectively reviewed.

    Results: There was no difference in preoperative cytologic classification between NIFTPs and invasive encapsulated FVPTCs, whereas infiltrative FVPTC was more likely to be Bethesda class VI than the encapsulated type (50% versus 9.5%; P = 0.033). Lymph node metastasis was not found in NIFTPs. There was no BRAFV600E mutation in NIFTPs, whereas one of 11 invasive encapsulated FVPTCs and three of 8 infiltrative FVPTCs harbored BRAFV600E. RAS mutations were frequently detected in encapsulated FVPTCs (5 of 10 NIFTPs and 4 of 11 invasive encapsulated FVPTCs) but were only detected in one case of the infiltrative type.

    Conclusions: There were no differences in molecular or clinicopathologic profiles between non-invasive and invasive encapsulated FVPTCs, except for lymph node metastasis and the presence of BRAFV600E. NIFTP has favorable pathologic characteristics with a high frequency of RAS mutations.


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